Nonwoven fabrics have become quite popular for many different end uses wherein textile-like properties, such as softness, drapeability, strength and abrasion resistance are desired. A very significant market for nonwoven fabrics, and in particular nonwoven webs including predominately textile-length fibers, is for facing sheets in products such as disposable diapers. These sheets are placed in direct contact with the baby's skin, and therefore, at least the surface of the nonwoven fabric contacting the skin should be extremely soft and nonabrasive to prevent chafing.
Of particular interest for use as facing sheets are carded non-woven webs having a low basis weight of no more than about 0.0339 kg/m.sup.2 (1 oz./yd.sup.2). A representative method of forming such a carded nonwoven web is disclosed in U.S. Pat. No. 3,772,107, issued to Gentile et al, and assigned to Scott Paper Company. This type of web is characterized by highly directional properties in view of the fact that the fibers tend to align in the direction of web formation. Although some fibers are rearranged into the cross-machine-direction during web formation, the fibrous web generally is considerably weaker in the cross-machine-direction than in the machine-direction.
Carded nonwoven webs commonly are stabilized by some type of bonding operation, with an effort being made to improve the cross-machine-direction wet tensile energy absorption level (CDWTEA) without creating harsh, abrasive or stiff characteristics that would make the webs unsuitable for use as a diaper facing sheet, or for that matter, for other uses wherein soft, nonabrasive surface characteristics are desired. Efforts to-date have met with moderate success. However, for future generation diapers, higher levels of softness, surface feel and drapeability are desired. These desired tactile properties need to be achieved in webs having the necessary strength and stretch characteristics to permit them to function adequately as a facing sheet. This is an extremely challenging objective since bonding the web to achieve the necessary strength and stretch characteristics (i.e. TEA) generally is accompanied by reduced, or impaired tactile properties.
Tensile energy absorption (TEA) is the area under the stress/strain curve at web failure, and represents the energy absorbed by the product as it is stretched to failure.
The TEA and strength levels reported in this application can be determined on a Thwing Albert Electronic QC Tensile Tester, "Intelect 500", with a 160 ounce load cell, and being set at 99% sensitivity. The test is carried out by clamping a 0.0254 m (1 inch).times.0.1778 m (7 inch) rectangular test sample in opposed jaws of the tensile tester with the jaw span being 5 inches. The jaws are then separated at a crosshead speed of 0.127 m (5 inches) minute until the sample fails. The digital integrator of the tensile tester directly computes and displays tensile strength (grams/inch), TEA (inch-grams/inch.sup.2) and stretch (%) at failure. Wet TEA, strength and stretch values are obtained by immersing the sample in water prior to testing.
One very desirable technique for stabilizing nonwoven webs is to employ a predominate amount of thermoplastic fibers in the construction, and then to autogenously bond the web structure by the application of heat and pressure to the web. Thus, in these webs the thermoplastic fibers actually constitute the bonding medium, and no additional binder needs to be added.
Many different arrangements have been suggested for autogenously bonding webs formed of thermoplastic fibers, as exemplified by U.S. Pat. Nos. 3,542,634 (Such et al); 3,261,899 (Coates); 3,442,740 (David); 3,660,555 (Rains et al); 3,855,046 (Hansen et al) 4,005,169 (Cumbers); 4,035,219 (Cumbers); 4,128,679 (Pohland); and 4,151,023 (Platt et al).
Both the Coates' U.S. Pat. No. (3,261,899) and the Hansen et al U.S. Pat. No. (3,855,046) suggest preheating the web prior to actually establishing the desired bond structure in a subsequent pressure bonding operation. Although Coates does broadly suggest infrared heating a web prior to passing it through a heated pressure nip (see Ex. V), there is no suggestion of controlling the bond structure through the web to achieve any particular balance of properties.
The Hansen et al U.S. Pat. No. (3,855,046) describes a web formed of thermoplastic continous filaments that is preheated by the same smooth-surfaced roll 30 that cooperates with the heated embossing roll 32 to establish the bonding nip. Thus, control of the preheating temperature independent of the bonding parameters cannot be achieved, since the temperature to which the smooth-surfaced roll 30 is heated must generally be balanced between the requirements for preheating on the one hand, and the requirements for establishing the desired bond structure. Even though other parameters can be varied to regulate the amount of preheating, such as controlling the amount of wrap of the web about the smooth-surfaced roll 30 upstream of the bonding nip, it is believed that the desired independent control of the preheating and bonding operations is extremely difficult to obtain with this type of arrangement. In fact, in forming low basis weight webs of less than about 0.0339 kg/m.sup.2 (1 oz./yd.sup.2) the bond structure on each side of the web is disclosed as being generally the same; having an unfused bond area coefficient (ubac) of less than about 65%. The high percentage of fused, or melt bonds, established in these latter webs is not believed to provide the necessary tactile characteristics (e.g., softness, drapeability, surface smoothness, etc.) being sought after in products such as new generation diaper facing structures.